20. Seeley's Dinosaur Dichotomy, 1888
Between 1866 and 1883, various authorities on dinosaurs, including Huxley, Cope, and Marsh, had produced classification schemes that attempted to bring order to the great variety of dinosaur specimens that had been and were being discovered. Marsh, in particular, had proposed to divide dinosaurs into four orders: sauropods, theropods, stegosaurs, and ornithopods; Cope had offered a different scheme. Most of the systems were based on foot structure, with teeth also taken into account.
But in a paper delivered in 1887 and published in 1888, Harry G. Seeley pointed out that the term dinosaur was being used for two rather different kinds of reptiles. There were those, like Marsh's theropods and sauropods, that had a pelvis with a forward protruding pubic bone. And there were those, like the stegosaurs and ornithopods, that had a divided pubis, with one branch extending backwards along the ischium. Seeley saw this as a critical difference, and the basis for a fundamental dichotomy. Since the backward-protruding pubis is characteristic of modern birds, he called this group Ornithischia, the bird-hipped dinosaurs, and gave it the status of an order. The sauropods and theropods were placed in another order, Saurischia, or lizard-hipped dinosaurs. In the line drawing which he printed with the paper, the top two figures represent the Ornithischia, and the bottom two the Saurischia. Note that it would have been impossible for Seeley to have conceived his scheme much earlier than he did, for the sauropod and stegosaur orders (reduced by Seeley to sub-orders) had only just been discovered (see items 18 and 19).
Seeley, Harry Govier. "On the Classification of the Fossil Animals commonly named Dinosauria," in: Proceedings of the Royal Society of London, vol. 43, (1888), pp. 165-171. This work was on display in the original exhibition as item 20.